Dual-tone multi-frequency (DTMF) signaling is an in-band telecommunication signaling system using the voice-frequency band over telephone lines between telephone equipment and other communications devices and switching centers. DTMF telephone keypads provide keys that each represent a digit, symbol, or letter used to convey a telephone number or other information or data. When a user presses a key on the DTMF telephone keypad, a DTMF symbol comprising a combination of two frequencies is sent over the telephone line.
A traditional automated telephone system is implicitly aware of what information is requested (e.g., telephone number to call), the number of expected symbols (e.g., 10) and other information. Because of this foreknowledge of the structure of the audio, a traditional DTMF decoder does not need to differentiate dialed telephone routing data (e.g., a phone number) from other dialed symbols contained within telephone audio content. However, there is a need, for example, in the law enforcement context, to be able to capture only the telephone routing data without capturing the non-routing symbols or other content within the telephone audio.